Shuttle Shifting for a Continuously Variable Transmission

ABSTRACT

A method for performing a shuttle shift with a continuously variable transmission of a work machine is disclosed. The method may generally include adjusting a swash plate angle of a hydrostatic power unit of the transmission in a first direction to reduce a travel speed of the work machine in an off-going direction, initiating a directional swap between an off-going directional clutch of the continuously variable transmission and an on-coming directional clutch of the continuously variable transmission while the work machine is traveling in the off-going direction and adjusting the swash plate angle of the hydrostatic unit in a second direction after the initiation of the directional swap to reduce slippage across the on-coming directional clutch, wherein the second direction is opposite the first direction.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims priority to U.S. ProvisionalPatent Application No. 61/527,455, filed on Aug. 25, 2011 and entitled“Shuttle Shifting for a Continuously Variable Transmission, thedisclosure of which is hereby incorporated by reference herein in itsentirety for all purposes.

FIELD OF THE INVENTION

The present subject matter relates generally to continuously variabletransmissions and, more particularly, to a system and methods forcontrolling a continuously variable transmission in order to provide forimproved shuttle shifting.

BACKGROUND OF THE INVENTION

Continuously variable transmissions utilizing a hydrostatic power unit,hereinafter sometimes referred to as hydro-mechanical continuouslyvariable transmissions, are well known. A variety of work machinesutilize this type of transmission for industries such as agriculture,earth moving, construction, forestry, and mining. In operation, thefluid displacement of the hydrostatic power unit is varied to change theoutput to input ratio of the transmission, that is, the ratio betweenthe rotating output of the transmission, and the input. This isaccomplished by varying the angle in a swash plate of a variabledisplacement fluid pump or motor of the hydrostatic unit. In a commonmode of operation referred to as a shuttle shift, the direction ofmovement of the machine is changed, often under load, a common exampleof which being when a tractor loader moves in one direction to pick orscoop up a load, then lifts the load and reverses direction, ofteninvolving a turning movement, and unloads the load. This sequence isthen reversed, and is often repeated many times. Sometimes, such shuttleshifting operations are performed on slopes or inclines. Such movementstend to subject elements of the transmission to wear and tear, and canraise the temperature of various elements, particularly clutches, andthus raise performance, longevity and reliability concerns. It is alsotypically desired for shuttle shifts to be completed relatively quicklyand seamlessly, with little or no jerking or lurching of the machine.

In one category of the transmissions, the hydrostatic power unit isconfigured such that to effect movement of the vehicle in one direction,a swash plate of the unit will be tilted in one direction. To effectvehicle movement in the opposite direction, the swash plate is tilted inthe opposite direction. When no vehicle movement is sought, e.g., noforward or rearward motion, the swash plate of the unit is moved to azero tilt angle or near zero angle. Then, to effect movement of thevehicle in one direction or the other, the swash plate is appropriatelytilted in the requisite direction to the requisite angle. In thiscategory of transmission, if multiple speed ranges are provided, zerospeed for each range will be the zero or near zero position, whichpresents no problem or limitation for shuttle shifting to move thevehicle in opposite directions.

However, another category of continuously variable transmissions,commonly used in a variety of heavy vehicles such as work machines,including for construction, earth moving, forestry, and agriculture,wherein shuttle shifting is commonly used, employs a hydrostatic powerunit configured such that at zero vehicle or machine speed, the swashplate of the hydrostatic power unit is at full displacement or near fulldisplacement, in one direction or the other, depending on the rangeselected, direction of travel and possibly other factors. Reference asan example in this regard, Weeramantry, U.S. Pat. No. 7,063,638 B2,issued Jun. 20, 2006. When shuttle shifting this type of transmission,the common practice is to reduce the gear ratio to achieve zero vehiclespeed, and then shift the transmission to move the machine in theopposite direction. When zero vehicle speed is reached, some time willbe required to move the swash plate to its new position, and during thistime the operator can apply a brake or engage a combination of opposingclutches to hold the wheels or tracks. However, a shortcoming of thismanner of shifting is that a delay can result as the swash plate ismoved. As another possible shortcoming, repeatedly performing shuttleshifts in the same manner can raise temperature related performance andreliability issues, particularly if the brake is repeatedly used todecelerate the vehicle or the same clutch is repeatedly used todecelerate and/or accelerate the vehicle during the shifts.Additionally, not all shuttle shifts are performed under the sameconditions, and it can be desirable to have alternative manners ofperforming a shuttle shift for the different conditions.

Thus, what is sought is a manner of overcoming one or more of thedisadvantages or shortcomings, and achieving one or more of the desiredcharacteristics, set forth above.

BRIEF DESCRIPTION OF THE INVENTION

Aspects and advantages of the invention will be set forth in part in thefollowing description, or may be obvious from the description, or may belearned through practice of the invention.

In one aspect, the present subject matter is directed to a method forperforming a shuttle shift with a continuously variable transmission ofa work machine, wherein the continuously variable transmission includesa hydrostatic power unit. The method may generally include adjusting aswash plate angle of the hydrostatic power unit in a first direction toreduce a travel speed of the work machine in an off-going direction,initiating a directional swap between an off-going directional clutch ofthe continuously variable transmission and an on-coming directionalclutch of the continuously variable transmission while the work machineis traveling in the off-going direction and adjusting the swash plateangle of the hydrostatic unit in a second direction after the initiationof the directional swap to reduce slippage across the on-comingdirectional clutch, wherein the second direction is opposite the firstdirection.

In another aspect, the present subject matter is directed to a methodfor performing a shuttle shift with a continuously variable transmissionof a work machine, wherein the continuously variable transmissionincludes a hydrostatic power unit. The method may generally includeadjusting a swash plate angle of the hydrostatic power unit to reduce atravel speed of the work machine in an off-going direction, initiating adirectional swap between an off-going directional clutch of thecontinuously variable transmission and an on-coming directional clutchof the continuously variable transmission while the work machine istraveling in the off-going direction and maintaining the swash plateangle constant after the initiation of the directional swap.

In a further aspect, the present subject matter is directed to a systemfor shifting a travel direction of a work machine from an off-goingdirection to an on-coming direction. The system may generally include acontinuously variable transmission having an off-going directionalclutch for engaging the continuously variable transmission in theoff-going direction and an on-coming directional clutch for engaging thecontinuously variable transmission in the on-coming direction. Thecontinuously variable transmission may further include a hydrostaticpower unit having swash plate angle that is adjustable in a firstdirection and a second direction, wherein the first direction isopposite from the second direction. In addition, the system may includea controller communicatively coupled to the continuously variabletransmission. The controller may be configured to adjust the swash plateangle in the first direction to reduce a travel speed of the workingmachine in the off-going direction. The controller may also beconfigured to initiate a directional swap between the off-goingdirectional clutch and the on-coming directional clutch while the workmachine is traveling in the off-going direction. Further, the controllermay be configured to adjust the swash plate angle in the seconddirection after the initiation of the directional swap to reduceslippage across the on-coming directional clutch.

These and other features, aspects and advantages of the presentinvention will become better understood with reference to the followingdescription and appended claims. The accompanying drawings, which areincorporated in and constitute a part of this specification, illustrateembodiments of the invention and, together with the description, serveto explain the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

A full and enabling disclosure of the present invention, including thebest mode thereof, directed to one of ordinary skill in the art, is setforth in the specification, which makes reference to the appendedfigures, in which:

FIG. 1 is a side view of a representative work machine including acontinuously variable hydro-mechanical transmission automaticallycontrollable according to the invention for selecting from alternativemanners of shuttle shifting and executing the selected shift;

FIG. 2 is a simplified schematic representation of the hydro-mechanicaltransmission of the work machine of FIG. 1;

FIG. 3 is a simplified diagrammatic representation of transmission ratioverses hydrostatic power unit ratio for the transmission of FIG. 2;

FIG. 4 is a simplified diagrammatic representation of the transmissionratio verses hydrostatic power unit swash plate angle for thetransmission of FIG. 2, for one of the selectable manners of shiftingthe transmission;

FIG. 5 is a simplified diagrammatic representation of the transmissionratio verses swash plate angle for the transmission of FIG. 2, foranother selectable manner of shifting the transmission;

FIG. 6 is a simplified diagrammatic representation of the transmissionratio verses swash plate angle for the transmission of FIG. 2, foranother selectable manner of shifting the transmission;

FIG. 7 is a simplified diagrammatic representation of the transmissionratio verses swash plate angle for the transmission of FIG. 2, for stillanother selectable manner of shifting the transmission;

FIG. 8 is a simplified diagrammatic representation of the transmissionratio verses swash plate angle for the transmission of FIG. 2, for stillanother selectable manner of shifting the transmission;

FIG. 9 is a simplified diagrammatic representation of the transmissionratio verses swash plate angle for the transmission of FIG. 2, for stillanother selectable manner of shifting the transmission;

FIG. 10 is a high level flow diagram showing steps of a method of theinvention for automatically selecting a manner of shuttle shiftingaccording to the invention;

FIG. 11 is a simplified diagrammatic representation of the transmissionratio verses swash plate angle for the transmission of FIG. 2, for yetanother selectable manner of shifting the transmission; and

FIG. 12 is a flow diagram showing steps of one embodiment of a methodfor performing a shuttle shift in accordance with the manner of shiftingshown in FIG. 11.

DETAILED DESCRIPTION OF THE INVENTION

Reference now will be made in detail to embodiments of the invention,one or more examples of which are illustrated in the drawings. Eachexample is provided by way of explanation of the invention, notlimitation of the invention. In fact, it will be apparent to thoseskilled in the art that various modifications and variations can be madein the present invention without departing from the scope or spirit ofthe invention. For instance, features illustrated or described as partof one embodiment can be used with another embodiment to yield a stillfurther embodiment. Thus, it is intended that the present inventioncovers such modifications and variations as come within the scope of theappended claims and their equivalents.

In general, the present subject matter is directed to a system andmethods for controlling a continuously variable transmission in order toprovide fast shuttle shifts with no stopping at zero speed and withreduced amounts of energy dissipated in the directional clutches. Inseveral embodiments, such shuttle shifts may be achieved bysimultaneously moving the swash plate of a hydrostatic power unit of thetransmission while slipping the on-coming directional clutch of thetransmission (i.e., gradually engaging the on-coming directional clutchas the direction of travel of the work machine is reduced/reversed).Such movement of the swash plate may generally allow for the speeddifferential across the on-coming directional clutch to be reduced,thereby reducing the amount of energy dissipated in such clutch. Thismay lead to energy savings by reducing the amount of energy required topump cooling oil to the on-coming directional clutch and may alsoprevent damage to the clutch due to overheating.

Referring now to the drawings, in FIG. 1, a representative vehicle inthe form of a work machine 1 is shown, which is a tractor representativeof those that can be used for a variety of uses, including, but notlimited to, agriculture, construction, earth moving and forestry. Workmachine 1 includes a power source 4 which will be, for instance, aninternal combustion engine, and is mechanically coupled to acontinuously variable hydro-mechanical transmission, a representativeembodiment 10 of which is shown schematically in FIG. 2. Transmission 10is automatically operable for selecting from several alternative mannersof performing shuttle shifts as a function of one or more monitoredconditions, and executing the selected shuttle shift according to theinvention, as will be explained.

Referring also to FIG. 2, transmission 10 is contained in a transmissionhousing 11 and includes a hydrostatic power unit 12 and a planetarypower unit 30 which are coupled to a driveline including a range gearset 58 mounted within transmission housing 11 and coupled to a load Lwhich here is the drive wheels of machine 1 as shown in FIG. 1. Itshould be understood that machine 1 can alternatively include a load Lthat comprises a track drive, or an operating system of the machine suchas but not limited to, a power take off (PTO).

Hydrostatic power unit 12 of transmission 10 includes a fluid pump 16coupled by fluid conduits 17 in a closed loop to a fluid motor 18. Motor18 is coupled to power source 4 via an input gear N6 and having anoutput gear N10. The power to the hydrostatic power unit 12 is providedby a driven gear N4 mounted on the forward shaft and engaged with gearN6. Output gear N10 is connected to ring gear NR of planetary power unit30 via gears N11 and N12.

Machine 1 includes a processor based controller 100 in connection withan input device 102 located preferably in operator cab 104 of machine 1,via a suitable communications path 108, to adjust the angle of a swashplate of pump 16 (swash plate denoted by a diagonal arrow through pump16), through a range of positions. As an exemplary embodiment, pump 16can be an electronically controlled variable displacement hydraulic pumpof well known construction.

Planetary power unit 30 includes a primary sun gear NS1 on a planetaryinput shaft 32 connectable with power source 4 via a forward directionalclutch 54 or a reverse directional clutch 52. Power unit 30 isselectively coupled to the load L, coupled to the hydrostatic power unit12 and selectively coupled to the power source 4, under automaticcontrol of controller 100. For connection to the load L, thehydro-mechanical transmission 10 includes an output shaft 60 coupled tothe load L which carries an input gear N18 engaged with an output gearN17 on a range ½ shaft of range gear set 58, and a gear N22 engaged witha gear N19 on a range ¾ shaft. The range ½ shaft can be coupled toplanetary power unit 30 via automatic operation of range selectors orclutches R1 and R2 for power flow through gears N13 and N14, or N15 andN16, respectively. The range ¾ shaft can be coupled to unit 30 via rangeselectors or clutches R3 and R4 for power flow via gears N13 and N20, orN15 and N21. Range ½ shaft and range ¾ shaft can also be simultaneouslycoupled to power unit 30, to provide dual power flow.

The control of the various clutches will be automatically controlled bycontroller 100, using actuators 106 connected to controller 100 viasuitable conductive paths 108. Transmission 10 also includes appropriatesensors, including pressure sensors 110 for sensing pressure conditionsin conduits 17 connecting pump 16 and motor 18, and speed sensors 112for sensing speeds of load shaft 60, all connected to controller 100 viaconductive paths 108. Controller 100 is also connected to engine 4 forreceiving speed and other information therefrom.

In operation, the continuously variable hydro-mechanical transmission 10can be operated to have a combined hydrostatic and mechanical power flowby engaging the reverse clutch 52 to power planetary power unit 30 viagears N1, N3, N5 and N7, or engaging forward clutch 54 to power it viagears N1, N8, and N2. It is also possible to operate transmission 10 fora pure hydrostatic power flow by disengaging both clutches 52 and 54.

As a result, with transmission 10, there is no selection for a workrange or road range per se. However, the transmission provides aseamless transition between ranges to provide work/road configurationsas desired. Speed change from zero to maximum speed is achieved in asmooth and continuous manner by changing the swash plate angle of thepump 16 under control of controller 100. For each speed range,substantially the full range of travel of the swash plate is used. Thatis, the swash plate will be at one end of the range of its travel forzero speed within the range, it will be at the other end for maximumspeed in that range, and the zero tilt or neutral position of the swashplate will be an intermediate position for the speed range, not the zerospeed position as it is for some other transmissions. This presents achallenge for execution of some transmission commands that require achange of state wherein the swash plate will have to be tilted to aposition significantly different from the present position, e.g., someshuttle shifts, as some time for the transition or movement to the newposition will be required. For other commands, e.g., shuttle shifts athigher speeds, the speed range will need to be changed, but it can beobserved that the required ending swash plate position is the same orsimilar to the beginning position, which presents an opportunity forshifting in a different manner than that for lower speed shifts.

Transmission 10 includes a parking brake 114 in connection with loadshaft 60, which is utilized according to the invention for enabling atleast one selectable manner of shuttle shifts. Parking brake 114 isconnected to controller 100 via a suitable conductive path 108 forautomatic operative control thereby, including to proportionally orgradually engage, and release or disengage, under certain conditions. Toachieve this latter capability, as a non-limiting example, parking brake114 can be controlled using a proportional pressure reducing valveoperated by an electrical signal from controller 100. For operation whenmachine 1 is not operating, parking brake 114 can be engaged by a springor other biasing element or elements, or by mechanical means.

Other conditions wherein parking brake 114 will be automaticallycontrolled by controller 100 to engage, or remain engaged if alreadyengaged, can include, but are not limited to, when power source 4 ofmachine 1 is turned off, the transmission is disengaged, the operatorleaves the operator seat, and if the FNR lever is left in F for acertain period of time without movement. Controller 100 will alsocontrol the parking brake to remain engaged when a command is receivedto disengage the parking brake, until certain conditions are met, aswill be explained. Other conditions include when a command is receivedvia an input device 102, e.g., FNR lever or the like, to change theoperating state of the transmission. Such commands can include a changeto, or in close proximity to, a neutral or zero movement state, or aclutch command.

It should be appreciated that the work machine 1 shown in FIG. 1 simplyillustrates one example of a suitable work machine 1 with which thedisclosed system and method may be utilized. Similarly, theconfiguration of the transmission 10 shown in FIG. 2 simply illustratesone example of a suitable transmission with which the disclosed systemand method may be utilized. Thus, one of ordinary skill in the artshould appreciate that application of the present subject matter neednot be limited to the particular work machine 1 and transmission 10shown in FIGS. 1 and 2, but, rather, the present subject matter may beadvantageously used with various types/configurations of works machinesand transmissions.

Referring also to FIG. 3, a graphical representation of the relationshipof transmission ratio, denoted TRR, to hydrostatic power unit ratio(motor speed/pump speed) denoted HRR, is shown, for the four selectableforward ranges, and four selectable reverse ranges of operation oftransmission 10: namely, forward range 1 or low (denoted FR1); forwardrange 2 (denoted FR2); forward range (FR3); forward range (FR4); reverserange 1 (RR1); reverse range 2 (RR2); reverse range 3 (RR3); and reverserange 4 (RR4). HRR directly relates to swash plate angle, which is theparameter controlled by controller 100. In FIG. 3, it should be notedthat for each of the ranges, the zero tilt position of the swash platelies between the maximum degrees of tilt in the opposite directions ofmovement of the swash plate. Thus, at the lowest hydrostatic power unitratio for forward range RR1, the swash plate will be at or near maximumtilt in the left hand direction as depicted, which is also the zerospeed ratio for the transmission for that direction, while at thehighest ratio for that range the swash plate will be at or near itsmaximum tilt in the opposite direction, which is the right handdirection as depicted. It can be noted that for the reverse direction,the opposite is true. Thus, it can also be observed that to go from zerospeed in the lowest range in the forward direction to zero speed in thelowest range in the reverse direction, the swash plate must travelsubstantially its entire range of movement, as depicted by distance ROM.It should also be noted that to engage reverse, not only must theforward and reverse directional clutches 54 and 52 be swapped, but theswash plate must be moved the distance ROM. Here, it should be notedthat when referring to the term “maximum” tilt, some marginal amount ofswash plate movement should still remain such that zero vehicle speedcan still be achieved under conditions such as, but not limited to,leakage in the hydrostatic power unit, that may cause the motor torotate more slowly for a given swash plate angle.

Additionally, while the swash plate is being moved from one side to theother, generally the driveline cannot be engaged, since this couldresult in higher speeds if the clutch is not slipped. There are perhapstwo main options to deal with this, one is to four square thetransmission (lock the output shaft) by applying both the R1 and R3clutches, and the second is to use the parking brake. If four squaringis used, it is difficult to control, since the swash plate movement isnot completely decoupled, and moving the swash plate tends to move thevehicle in the opposite direction, and this must be compensated for bycontrolling the pressure in either the R3 or R1 clutches.

As an advantage of the present invention, shuttle shifting shall beallowed from any forward speed to any reverse speed. According to theinvention, shuttle shifts will have three phases. During the first,machine 1 is decelerated using the swash plate, with the decelerationlimited to a target value. Next, the forward and reverse clutches 54, 52are swapped. Directional swapping is defined as the part of the shuttleshift from when the off-going directional clutch starts to dump to whenthe on-going clutch is finished ramping up and is fully engaged. Thelast phase of a shuttle is when the machine may be accelerated using theswash plate to the final speed in the opposite direction. This is donewith the swash plate, range shifting as needed, and limited to thedesired transmission acceleration value. It should also be noted thatdeceleration is controlled in all phases of all types of shuttles,during the ratio changing, deceleration with the parking brake, anddeceleration then reacceleration using clutch slipping.

As a consideration, it is advisable to minimize energy dissipated byclutches to prevent damage. It has been found that one of the best waysto do this is to reduce the speed of the vehicle prior to the shift.Directional swapping is always done in the first range. If the speed ishigher when the shuttle shift is commanded, the vehicle will be slowedby normal swash plate movement and range shifting. As a result, in theinvention, both the speed when the shuttle is commanded (or the currentspeed) and the final opposite speed will be needed to determine when andhow to swap the clutches and move the swash plate.

As another consideration, as evidenced by the distance ROM, shuttleshifting for transmissions, such as transmission 10, is challengingbecause the swash plate may need to move a considerable distance beforethe on-coming clutch can be engaged, or the vehicle may go too fastbefore the swash plate reaches its final position. In this case, it hasbeen found that it is best to apply parking brake 114, to keep thevehicle from rolling while in neutral when the swash plate is beingmoved. As another consideration, since the time to move the swash platemay vary considerably, and engaging the on-coming clutch while the swashplate is not in position can cause overspeed conditions, controller 100should fill the on-coming clutch, and then wait until the motor speed(swash plate error) has reached it proper value before engaging theon-coming clutch, to achieve consistent shifts. During shuttle shifts,the desired transmission output acceleration (DTOA) is desirablyachieved through all phases, and especially needs to be matched duringtransitions between phases. The pressure in the on-coming clutch shouldbe carefully controlled to achieve the correct DTOA, both throughinitialization to the proper pressure and closed loop control. If theparking brake is used for decelerations, it is also controlled in aclosed loop fashion to achieve DTOA.

Referring also to FIGS. 4 through 9, several manners of shuttleshifting, for different respective conditions, will be explained. Inthese FIGURES, the vertical axis represents the ratio of thetransmission output speed to the engine speed, denoted TRR, and is alsorepresentative of the vehicle speed of movement in opposite directions(forward above horizontal axis; reverse below). The horizontal axisrepresents the swash plate angle of the hydrostatic power unit. In thegraphs a forward-to-reverse shuttle shift is depicted, but thedescription will also apply to a reverse-to-forward shuttle shift forthe applicable conditions. In this regard, FIG. 4 depicts a manner ofshuttle shifting for a low forward beginning speed, and a low reverseending speed. This utilizes automatic operation of the parking brakejust as the vehicle is brought down to zero speed and the range clutchis dumped.

As a first step, the speed is reduced by moving the swash plate, asdenoted by distance D1. In FIG. 4, range shifts are not shown, but ifthe shuttle shift is commanded from a higher speed range, then the swashplate will be moved and the range shifts will occur just as they do innormal speed changes. Just like normal speed changes, the rate of changeof the desired transmission ratio may be limited and adjusted by controlsoftware of controller 100.

As the vehicle reaches zero speed, the range clutch is dumped, andparking brake 114 is automatically applied to reduce required operatoraction, e.g., clutching and application of the service brake, to preventunwanted movements of the vehicle. The applied pressure of the parkingbrake should be high enough to keep the vehicle from moving in the wrongdirection, even on a steep hill. The swash plate will then be moved overdistance ROM to reverse tilt. During movement of the swash plate overdistance ROM, the on-coming directional clutch is filled. Then, afterthe swash plate is moved to the correct position and the on-comingdirectional clutch is filled, the parking brake will be released ordisengaged and the vehicle will begin to move. At a selected time, e.g.,at the end of the ROM, the directional swap will occur (on-comingdirectional clutch is engaged and the off-going directional clutch isdumped), and the swash plate is moved in a manner to achieve theselected reverse speed.

Another manner of shuttle shifting according to the invention isillustrated in FIG. 5 is a constant SPA shuttle shift. This manner isapplicable for high speed to high speed shifts, and also high to low andlow to high shifts (FIG. 6). Note that this manner of shuttle shift canbe utilized at almost any swash plate angle, depending on the finalreverse speeded needed. The energy will depend on the squared differencein the speed across the on-coming clutch as it engages. Changing theswash plate angle to slow the vehicle before performing the clutch swapwill reduce the energy and probably result in better performance. Theenergy dissipated will be similar to the case of high speed to highspeed shift. In FIG. 5, range shifts are not shown, but if the shuttleshift is commanded from a higher speed range, the swash plate will bemoved and the range shifts will occur just as they do in normal speedchanges.

Next, when the transmission ratio is at a given point, the directionalclutches are swapped and the swash plate is moved to a value for aparticular transmission ratio in the opposite direction. The on-comingdirectional clutch is filled in anticipation of this point. This swapmay be initiated such that the swash plate angle either continues changein the same direction slightly, is held constant during the swap, oractually reverses direction during the swap, depending on the relativevalues of the various parameters. Reversing the direction of the swashplate angle during the swap can result in less energy being dissipatedin the clutch, which is desirable, but if the swash plate control issluggish compared with the time needed for the swap, it may be better tohave some movement of the swash plate in the same direction during theswap. Perhaps more importantly, moving the swash plate during the swapcreates a reaction torque that affects the deceleration, so consistentdecelerations are easier to achieve if the swash plate movement isminimized. However, as will be described below with reference to FIGS.11 and 12, it may be desirable in many instances to move the swash plateduring the swap (e.g., by reversing the direction of the swash plateduring the directional swap). In such instances, steps may be taken tocontrol or minimize the reaction torque created as a result of any swashplate angle adjustments occurring during the directional swap.

FIG. 6 illustrates a high to low speed shuttle shift in the justdescribed manner. This illustrates that shuttles that don't require theswash plate to move back in the opposite directional don't necessarilyneed to be high speed to high speed ones. The shift occurs at speedshigher than for the high to high speed shift, since the reverse speed isslower. Note that these types of shifts can occur at most any swashplate angle, depending on the final reverse speeded needed. The energywill depend on the squared difference in the speed across the on-comingclutch as it engages.

Medium speed shuttle shifts are ones where generally there is enoughtime to move the swash plate into position before the vehicle comes to astop, although this may not always be the case. The proper time toswitch between the shuttle shift strategy using the parking brake todecelerate described here and the shuttle shift using ratio controlstrategy described above can be determined by which feels better intesting.

As illustrated in FIG. 7, when the shuttle shift using the parking braketo decelerate is initiated, there will be a slight delay as the parkingbrake is prepared to be applied (this cannot be done in advance, sincethere is no ratio changing before the swap). The off-going clutch isdumped, since the range swap must be performed, but the range clutchalso must be dumped to decouple the planetary from the wheels and avoidany torque from moving the swash plate affecting the deceleration. Theparking brake is then used to decelerate the vehicle while the swashplate is moved into position and the on-coming clutch is engaged.Engaging the on-coming clutch does not affect the output torque, sincethe range clutch remains disengaged. Generally, the swash plate is inposition before zero speed is reached (since lower speed shuttles don'tuse this method), and the vehicle will not stop at zero, but this maynot be the case if the swash plate movement is slower than normal. Assoon as the swash plate is in position, the on-coming clutch is used tocontinue the deceleration to zero and reaccelerate in the oppositedirection.

Shuttle shifts may also comprise combinations of the types describedabove, as illustrated in FIGS. 8 and 9. Shifts may use the parking braketo decelerate to zero, then use the ratio changing to reaccelerate, ifthe final speed needed is low. Similarly, if the initial speed is low, ashuttle shift may use ratio changing to slow the vehicle to zero, thenengage the range clutch to take off to a higher speed. The exact speedat which the controller change approaches from the shuttle shift methodusing the parking brake to decelerate and the method using ratio controlis determined by tuning or experimentation, and as the shuttle shiftsusing the parking brake to decelerate are improved (perhaps throughfaster swash plate movement), the speed may be lowered. At some point,the ratio changing is smoother than deceleration with the parking brake.Generally it is not as smooth to let the vehicle actually come to a stopwith the shuttle shift method using the parking brake to decelerate.

If the directional clutches, range clutches or parking brake are toohot, e.g., according to a sensed temperature value or values, or anestimate of the temperature based on the history of clutch pressures andworst case assumptions on the clutches, then controller 100 can inhibitthe shuttle logic directional swap, and use ratio changing to bring thevehicle to a stop. If the ratio changing is not effective, there willnot be a timeout, the system will continue to wait for the vehicle toslow down, then complete the shuttle.

If the directional swap is in progress and the clutches become too hot(perhaps more typical than starting hot), then a “reverting” logic isused. This includes setting the desired transmission ratio to thecurrent transmission ratio, so the swash plate will be moved to what isneeded for re-engaging. Note that the clutches are simply re-engaged andthe direction swap is over, regardless of the transmission ratio, orhydrostatic power system ratio. The swash plate will then be moved toreduce the transmission ratio to zero, and then the angle reversed, andthen positioned for the target speed in the new direction.

It should be noted that if an operator commands a shuttle shift, and thevehicle does not slow down fast enough, or does not slow at all, such aswhen pulling a trailer down a hill, it is advisable and normal for theoperator to use the service brakes (typically brake pedals on the floorof the operator cab). The service brakes can always be used duringshuttle shifts to increase deceleration.

Referring also to FIG. 10, a high level flow diagram of steps of amethod of the invention for controlling shuttle shifts is shown. In thediagram, once the commands for a shuttle shift are received, as denotedin block 150, it is determined whether a high temperature conditionexists in the parking brake or clutches, as denoted at block 152. Ifyes, a ratio controlled shift is selected, as denoted at block 154, andthe shift is executed, as denoted at block 156. If at block 152 no hightemperature condition is present, it will be determined if at least oneof the start and end speeds are greater than thresholds, a high to high,high to low, or low to high speed shift, as denoted at block 158. Ifyes, a constant SPA shuttle shift is utilized, as illustrated in FIGS. 5and 6, and denoted at block 160, and the shift executed. If at block 158at least one of the speeds is not above the threshold values, a shuttleshift using the parking brake to decelerate will be utilized, as denotedat block 162 and illustrated in FIGS. 4 and 7, and the shift executed asdenoted at block 156. This can be a shuttle shift using ratio control ora shuttle shift using the parking brake to decelerate. If, duringexecution of the shift, a high temperature condition is detected, asdenoted at decision block 164, the shift in process will be converted toa ratio control shift (if not already that type), as denoted at block154, execution will proceed in that manner. When the shift is complete,the logic will return to block 150, as denoted by decision block 166.

Referring now to FIG. 11, another manner of shuttle shifting that may beapplicable for high speed to high speed shifts (FIG. 5) and also high tolow and low to high shifts (FIG. 6) is illustrated in accordance withaspects of the present subject matter. However, unlike the manner ofoperation shown in FIGS. 5 and 6 in which the swash plate angle is heldconstant, the illustrated shuttle shift requires that the angle of theswash plate be adjusted during the directional swap. Specifically, asshown in FIG. 11, the direction of movement of the swash plate may bereversed after the initiation of the directional swap in order to reducethe speed differential across the on-coming directional clutch, therebyallowing for the amount of energy dissipated in the on-comingdirectional clutch to be reduced.

Initially, the shuttle shift may be performed similarly to the shuttleshift shown above in FIGS. 5 and 6. Specifically, after receipt of ashuttle shift command (indicated by point 202 in FIG. 11), e.g., byreceiving an operator input from input device 102 (FIG. 2), the TRR orvehicle speed of the work machine 1 may be reduced by moving the swashplate and making any required range/ratio shifts. For example, as shownin FIG. 11, the swash plate angle may be adjusted in a first direction(indicated by arrow 204 in FIG. 11) to reduce the vehicle speed of thework machine 1. Such deceleration of the work machine 1 may generallyallow for a reduction in the energy dissipated in the directionalclutches during the directional swap.

In addition, while the swash plate angle is being adjusted, theon-coming directional clutch may be filled in anticipation of thedirectional swap. For instance, in the illustrated embodiment, aforward-to-reverse shuttle shift is being performed and, thus, thereverse directional clutch 52 (FIG. 2) may be pre-filled with hydraulicfluid while the swash plate angle is being adjusted. As such, thereverse directional clutch 52 may begin to be gradually engaged when theTRR reaches the point at which the directional swap is initiated(indicated by point 206 in FIG. 11).

It should be appreciated that, since FIG. 11 illustrates aforward-to-reverse shuttle shift, the first direction 204 corresponds toa right-to-left (or positive-to-negative) adjustment of the swash plateangle, which, as shown in FIG. 3, provides for a reduction of the TRR orvehicle speed in forward range 1 (denoted FR1). However, in areverse-to-forward shuttle shift, the first direction 204 may correspondto a left-to-right (or negative-to-positive) adjustment of the swashplate angle, which, as shown in FIG. 3, provides for a reduction of theTRR or vehicle speed in reverse range 1 (denoted RR1).

It should also be appreciated that a variety of different factors may beused to determine the point 206 at which the directional swap may beinitiated. For example, when performing the shuttle shift shown in FIG.11, the TRR must be within a predetermined range (e.g., from about 0.15to about 0.1) before the directional swap may be initiated. However,when performing the shuttle shift shown in FIG. 4, the TRR may be withina different range (e.g., less than about 0.02) before the directionalswap may be initiated. In addition, the pressure within the on-comingdirectional clutch must be increased a sufficient amount so that theclutch can respond quickly and accurately to the control signalsinitiating the swap. Moreover, various other operating conditions of thework machine 1 may also be checked to ensure that the directional swapmay be initiated, such as that the off-going directional clutch isactually engaged prior to the swap and that there are no faults withinthe control logic.

Once the TRR reaches point 206, the directional swap is initiated.Specifically, at point 206, the off-going directional clutch (e.g.,forward directional clutch 54) may be immediately dumped or disengaged.In addition, the on-coming directional clutch (e.g., reverse directionalclutch 52) may be gradually engaged. For instance, the pressure of thehydraulic fluid supplied to the on-coming directional clutch may begradually increased such that the on-coming directional clutch ispartially engaged (i.e., slipping) as the off-going directional clutchis disengaged. As will be described below, the hydraulic pressure withinthe on-coming directional clutch may continue to be gradually increasedas the swash plate angle is adjusted until the on-coming directionalclutch is fully engaged (i.e., such that no slippage occurs across theon-coming directional clutch).

It should be appreciated that, as shown in FIG. 11, the directional swapmay be initiated while the work machine 1 is still traveling in theoff-going direction (e.g., the forward direction). Thus, the off-goingdirectional clutch may be disengaged and the on-coming directionalclutch may begin to be engaged prior to the work machine 1 stopping orotherwise reversing its travel direction. By initiating the directionalswap while the work machine is still traveling in the off-goingdirection, the shuttle shift may be performed without stopping ortemporarily pausing the work machine 1 at zero speed.

Additionally, after the directional swap is initiated, the direction inwhich the swash plate angle is being adjusted may be reversed.Specifically, as shown in FIG. 11, the swash plate angle may be adjustedin a second, opposite direction (indicated by arrow 208) as the traveldirection of the work machine 1 shifts from the off-going direction(e.g., the forward direction) to the on-coming direction (e.g., thereverse direction). Such reversing of the swash plate may generallyallow for the speed differential across the on-coming directional clutchto be reduced, thereby reducing the amount of energy dissipated in theon-coming clutch during the shuttle shift.

As the slippage across the on-coming directional clutch gets low, themovement of the swash plate may be slowed and subsequently reversed.Specifically, as shown in FIG. 11, the swash plate angle may be adjustedin the second direction 208 until the amount of slippage across theon-coming directional clutch falls below a predetermined slip threshold(indicted by point 210). At this point 210, the rate of change of theswash plate angle may be slowed and eventually stopped to allow thedirection of movement of the swash plate to be reversed from the seconddirection 208 back to the first direction 204. As such, when theslippage across the on-coming directional clutch goes to zero (i.e.,when the on-coming directional clutch is fully engaged), the swash platemay be moving in the appropriate direction and at the appropriate rateto allow for a seamless transition. The swash plate may then be moved inthe first direction and any necessary range/ratio changes may be made toaccelerate the work machine to the desired final speed (indicated bypoint 212.

It should be appreciated that the predetermined slip threshold maygenerally be determined based on the actual or expected rate at whichthe slippage across the on-coming directional clutch may be reducedand/or the actual or expected rate at which the swash plate angle may beadjusted. Specifically, as indicated above, it may be desirable for themovement of the swash plate to be completely reversed by the time theon-coming directional clutch is fully engaged. Thus, the predeterminedslip threshold may be selected such that sufficient time is provided forreversing the direction of movement of the swash plate prior to theslippage across the on-coming directional clutch being reduced to zero.

Additionally, as indicated above, although reversing the direction ofthe swash plate provides for a reduction in the energy dissipated in theon-coming directional clutch, such movement of the swash plate alsoresults in a reaction torque. In particular, moving the swash platewhile both the driveline and the on-coming directional clutch areengaged generates a reaction torque that adds to the torque transmittedthrough the on-coming clutch, which can cause a reduction in thedeceleration of the work machine 1. However, in several embodiments, theeffect of the reaction torque may be mitigated by carefully regulatingthe hydraulic pressure within the on-coming directional clutch as themovement of the swash plate reverses direction (e.g., from point 206 toa point at which the swash plate is moving in the second direction 208at a steady speed). In one embodiment, the pressure within the on-comingdirectional clutch may be controlled as a function of a rate of changeof the transmission ratio (denoted TRR) of the transmission 10. Forinstance, the rate of change of TRR may be continuously monitored andcompared to a target deceleration for the transmission 10. The targetdeceleration may generally correspond to a control setting for limitingthe deceleration rate of the transmission 10 during shuttle shifting andmay be controlled by a number of factors including, but not limited to,a user setting for “aggressiveness” (low, medium and high). If the rateof change of TRR varies from the target deceleration, the pressurewithin the on-coming directional clutch may be adjusted until the targetdeceleration is achieved.

In addition, the reaction torque may also be counteracted by delayingthe movement of the swash plate in the second direction 208 until thepressure in the on-coming directional clutch is increased (e.g., bycontinuing to adjust to the swash plate angle in the first direction 204for a period of time after the initiation of the directional swap).Specifically, as shown in FIG. 11, at the point at which the off-goingdirectional clutch is disengaged and the on-coming directional clutchbegins to be engaged, the rate of change of the swash plate angle in thefirst direction 204 may be slowly reduced until the motion of the swashplate is momentarily stopped (indicated by point 214 in FIG. 11). Thiscontrolled reduction in the rate of change of the swash plate angle inthe first direction 204 may generally allow for the hydraulic pressurewithin the on-coming directional clutch to be ramped up a significantamount prior to reversing the direction of the swash plate, therebycounteracting the reaction torque generated during the shuttle shift.Thereafter, the reaction torque may be controlled by controlling therate of change of the swash plate angle in the second direction 208(e.g., as a function of a rate of change of the TRR).

Referring now to FIG. 12, a simplified flow diagram of one embodiment ofa method 300 for performing the shuttle shift described above withreference to FIG. 11 is illustrated in accordance with aspects of thepresent subject matter. As shown, in 302, the swash plate angle may beadjusted in a first direction. For example, as indicated above, theswash plate angle may be adjusted in the first direction 204 in order toreduce the speed of the work machine 1 in the off-going direction.Additionally, in 304, a directional swap may be initiated between theoff-going and on-coming directional clutches. Specifically, theoff-going directional clutch may be disengaged while the on-comingdirectional clutch may be gradually engaged. Moreover, in 306, the swashplate angle may continue to be adjusted in the first direction 204immediately after the initiation of the directional swap. For instance,as indicated above, the swash plate angle may be temporarily moved inthe first direction 204 after the initiation of the directional swap tocontrol the reaction torque generated during the shuttle shift. Further,in 308, the direction of movement of the swash plate may be reversedfrom the first direction 204 to the second direction 208. In doing so,the speed differential across the on-coming directional clutch may bereduced, thereby reducing the amount of energy dissipated in theon-coming directional clutch during the shuttle shift. In addition, in310, the direction of movement of the swash plate may be reversed backto the first direction 204. Specifically, as indicated above, themovement of the swash plate may be reversed after the amount of slippageacross the on-coming directional clutch falls below a predetermined slipthreshold, thereby allowing the swash plate to be moving in theappropriate direction and at the appropriate rate when the on-comingdirectional clutch is fully engaged.

It should be appreciated that the various method elements or steps ofthe disclosed method 300 may generally be implemented by the controller100 of the work machine 1. As indicated above, the controller 100 maygenerally comprise a processor-based device. Thus, in severalembodiments, the controller 100 may include one or more processor(s) andassociated memory device(s) configured to perform a variety ofcomputer-implemented functions. As used herein, the term “processor”refers not only to integrated circuits referred to in the art as beingincluded in a computer, but also refers to a controller, amicrocontroller, a microcomputer, a programmable logic controller (PLC),an application specific integrated circuit, and other programmablecircuits. Additionally, the memory device(s) of the controller 100 maygenerally comprise memory element(s) including, but are not limited to,computer readable medium (e.g., random access memory (RAM)), computerreadable non-volatile medium (e.g., a flash memory), a floppy disk, acompact disc-read only memory (CD-ROM), a magneto-optical disk (MOD), adigital versatile disc (DVD) and/or other suitable memory elements. Suchmemory device(s) may generally be configured to store suitablecomputer-readable instructions that, when implemented by theprocessor(s), configure the controller 100 to perform variouscomputer-implemented functions.

This written description uses examples to disclose the invention,including the best mode, and also to enable any person skilled in theart to practice the invention, including making and using any devices orsystems and performing any incorporated methods. The patentable scope ofthe invention is defined by the claims, and may include other examplesthat occur to those skilled in the art. Such other examples are intendedto be within the scope of the claims if they include structural elementsthat do not differ from the literal language of the claims, or if theyinclude equivalent structural elements with insubstantial differencesfrom the literal languages of the claims.

What is claimed is:
 1. A method for performing a shuttle shift with acontinuously variable transmission of a work machine, the continuouslyvariable transmission including a hydrostatic power unit, the methodcomprising: adjusting a swash plate angle of the hydrostatic power unitin a first direction to reduce a travel speed of the work machine in anoff-going direction; initiating a directional swap between an off-goingdirectional clutch of the continuously variable transmission and anon-coming directional clutch of the continuously variable transmissionwhile the work machine is traveling in the off-going direction; andadjusting the swash plate angle of the hydrostatic unit in a seconddirection after the initiation of the directional swap to reduceslippage across the on-coming directional clutch, wherein the seconddirection is opposite the first direction.
 2. The method of claim 1,wherein the off-going direction comprises a forward travel direction ora reverse travel direction of the work machine.
 3. The method of claim1, wherein initiating a directional swap between an off-goingdirectional clutch of the continuously variable transmission and anon-coming directional clutch of the continuously variable transmissioncomprises: disengaging the off-going directional clutch while the workmachine is traveling in the off-going direction; and gradually engagingthe on-coming directional clutch.
 4. The method of claim 3, furthercomprising temporarily continuing to adjust the swash plate angle in thefirst direction after the initiation of the directional swap.
 5. Themethod of claim 3, wherein gradually engaging the on-coming directionalclutch comprises controlling a hydraulic pressure within the on-comingdirectional clutch as a function of a rate of change of a transmissionratio of continuously variable transmission.
 6. The method of claim 3,wherein gradually engaging the on-coming directional clutch comprisesreducing an amount of slippage occurring across the on-comingdirectional clutch as a travel direction of the work machine shiftsbetween the off-going direction and an on-coming direction.
 7. Themethod of claim 6, wherein adjusting the swash plate angle of thehydrostatic unit in a second direction after the initiation of thedirectional swap to reduce slippage across the on-coming directionalclutch comprises adjusting the swash plate angle in the second directionuntil the slippage across the on-coming directional clutch falls below apredetermined slip threshold.
 8. The method of claim 7, furthercomprising temporarily continuing to adjust the swash plate angle in thesecond direction after the slippage across the on-coming directionalclutch falls below the predetermined slip threshold.
 9. The method ofclaim 7, further comprising adjusting the swash plate angle in the firstdirection after the slippage across the on-coming directional clutchfalls below the predetermined slip threshold.
 10. The method of claim 1,wherein adjusting the swash plate angle of the hydrostatic unit in asecond direction after the initiation of the directional swap to reduceslippage across the on-coming directional clutch comprises adjusting theswash plate angle in the second direction to reduce a speed differentialacross the on-coming directional clutch.
 11. The method of claim 1,wherein adjusting the swash plate angle of the hydrostatic unit in asecond direction after the initiation of the directional swap to reduceslippage across the on-coming directional clutch comprises adjusting theswash plate angle in the second direction as a function of atransmission ratio of the continuously variable transmission.
 12. Amethod for performing a shuttle shift with a continuously variabletransmission of a work machine, the continuously variable transmissionincluding a hydrostatic power unit, the method comprising: adjusting aswash plate angle of the hydrostatic power unit to reduce a travel speedof the work machine in an off-going direction; initiating a directionalswap between an off-going directional clutch of the continuouslyvariable transmission and an on-coming directional clutch of thecontinuously variable transmission while the work machine is travelingin the off-going direction; and maintaining the swash plate angleconstant after the initiation of the directional swap.
 13. A system forshifting a travel direction of a work machine from an off-goingdirection to an on-coming direction, the system comprising: acontinuously variable transmission including an off-going directionalclutch for engaging the continuously variable transmission in theoff-going direction and an on-coming directional clutch for engaging thecontinuously variable transmission in the on-coming direction, thecontinuously variable transmission further including a hydrostatic powerunit having a swash plate angle that is adjustable in a first directionand a second direction, the first direction being opposite the seconddirection; and a controller communicatively coupled to the continuouslyvariable transmission, the controller being configured to adjust theswash plate angle in the first direction to reduce a travel speed of theworking machine in the off-going direction, the controller being furtherconfigured to initiate a directional swap between the off-goingdirectional clutch and the on-coming directional clutch while the workmachine is traveling in the off-going direction, wherein the controlleris configured to adjust the swash plate angle in the second directionafter the initiation of the directional swap to reduce slippage acrossthe on-coming directional clutch.
 14. The system of claim 13, whereinthe controller is configured to initiate the directional swap bydisengaging the off-going directional clutch and gradually engaging theon-coming directional clutch.
 15. The system of claim 14, wherein thecontroller is configured to gradually engage the on-coming directionalclutch by controlling a hydraulic pressure within the on-comingdirectional clutch as a function of a rate of change of a transmissionratio of continuously variable transmission.
 16. The system of claim 13,wherein the controller is configured to temporarily adjust the swashplate angle in the first direction immediately after the directionalswap.
 17. The system of claim 13, wherein the controller is configuredto adjust the swash plate angle in the second direction until slippageacross the on-coming directional clutch falls below a predetermined slipthreshold.
 18. The system of claim 17, wherein the controller isconfigured to temporarily adjust the swash plate angle in the seconddirection immediately after the slippage across the on-comingdirectional clutch falls below a predetermined slip threshold.
 19. Thesystem of claim 17, wherein the controller is configured to adjust theswash plate angle in the first direction after the slippage across theon-coming directional clutch falls below the predetermined slipthreshold.
 20. The system of claim 13, wherein the hydrostatic powerunit includes a motor and a pump, the swash plate angle being adjustablewithin the pump.